ISTANBUL 

Russian President Vladimir Putin on Friday hailed the "heroic" North Korean soldiers who fought in Ukraine in a congratulatory letter to North Korean leader Kim Jong Un.

"The bonds of militant friendship, goodwill and mutual aid, which were consolidated in the days of the war long ago, remain solid and reliable even today," said Putin, referring to the end of Japan's 35 years of colonial rule in the Korean Peninsula.

This was "fully proved by the heroic participation of the DPRK (Democratic People's Republic of Korea) soldiers in liberating the territory of the Kursk Region from the Ukrainian occupationists," according to the letter published on the Korean Central News Agency (KCNA).

Kim and Putin last year signed a comprehensive strategic partnership agreement, requiring their militaries to support each other in case of a third-party attack on either of the two, which Putin said, in the letter, would continue to promote "the strengthening of Russia-DPRK cooperation in all spheres."

Pyongyang has not revealed the number of its soldiers deployed to the Russian frontlines against Ukraine, but South Korea has claimed North Korea sent thousands of its personnel.

Bodies of many North Korean soldiers, believed to have died in the ongoing Moscow-Kyiv war, were returned to Pyongyang.